This invention relates to printed circuit boards of laminated thermosetting sheets with epoxy resin as the bonding agent, as well as to a method for manufacturing such printed circuit boards. Printed (circuit) boards, especially multilayer printed boards, are the supporting base elements for wiring active and passive components. The operability of the printed board is, therefore, a decisive factor for the quality and reliability of integrated circuits and assemblies.
However, it must be noted that the operability of printed boards of the above-mentioned type is affected adversely by a process called copper migration. In that process copper bridges develop within relatively short times, particularly between through contacts and a circuitry layer potential position, which lead to a short and, therefore, to the failure of the corresponding assembly. These copper bridges are also called "conductive anodic filaments" (hereinafter CAF).
For the purpose of investigating the process of copper migration, laboratory methods have been developed in order to examine the formation of the filaments with time compression methods and, thereby, to find the causes for this process; and in addition to develop a process for determining the CAF proneness of different resin systems. Generally, test pieces are subjected here to a test voltage of about 100 V at elevated temperature and air humidity. In the printed boards predominantly used today of Class FR-4, having a base of glass fiber reinforced epoxy resins, for instance, the foregoing conditions lead to first failures after only a few days.
In practice, printed boards so damaged are rarely repaired since the short circuit bridges can be located only with difficulty. Rather in most cases the entire assembly is replaced, which generally is connected with significant costs. Inhibition of copper migration in printed boards therefore has increasing importance, especially with increasing minaturization and increased packing density, and, in addition, particularly with respect to the use of printed boards in humid places at elevated temperatures, for instance, in climatic zones associated with elevated moisture and heat.
The problem of copper migration in printed board materials of epoxide woven glass fabric laminate sheets, i.e., in printed boards of glass fiber reinforced epoxy resins has been known for years. Attempts at solving this problem have been concentrated to date on locating and testing CAF-resistant resin systems. In this connection, for instance, the use of reaction resins with a basis of triazines, bismaleimide-triazines (so-called BT resins) and polyimides instead of epoxy resins has been proposed. (See, J. P. Mitchell, T. L. Welsher in "Proceedings of the Printed Circuit World Convention II", vol. 1, pages 80 to 93, Eugen G. Leuze Verlag, 1981.) All systems with good CAF resistance such as, for instance, polyimides, however, have the disadvantage that they are substantially more expensive than epoxy resins. Since with improved semiconductor technology, the component costs drop and, hence, the share of the printed boards in the total cost of the assemblies is increasing, the use of resins of the type mentioned is not an economically sound solution to the problem of copper migration.
Thus, it is an object of this invention to provide high-quality and cost-effective printed boards with a basis of laminated thermosetting sheets with epoxy resins as the bonding agent, which have sufficient CAF resistance.